Why ntp client rejecte time server, even if time server is trusted resource?

Issue

• Have configured two or more ntp servers. Why some of the ntp clients sync properly (adding * & + sign before servers) and some of the clients reject (do not show * or + sign) against them?

[root@example1.com]# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*192.168.2.3    128.135.14.45   2 u  118 1024  377    1.113   -0.573   0.190
 192.168.2.4    116.137.150.44   3 u  593 1024  377    1.002   39.481   0.081

 [root@example2.com]#ntpq -p
  remote           refid      st t when poll reach   delay   offset    disp
==============================================================================
*192.168.2.3    128.135.14.45   2 u  118 1024  377     1.16  -18.952    1.62
+192.168.2.4    116.137.150.44   3 u   60 1024  377     1.42   21.671    0.98